Experimental investigation on air-side heat transfer and flow characteristics of longitudinal compact arrangement tube bundle with various transverse pitch ratios

The longitudinal compact arrangement (LCA) tube bundle shows great potential for advanced compact heat exchangers, with its excellent properties of low flow resistance and compactness. However, the lack of specific performance data and reliable empirical correlations hinders its practical application. This study experimentally investigates the airside heat transfer (Nusselt number, Nu) and flow resistance (friction factor, f) characteristics of LCA tube bundles across a wide range of transversal pitch ratios (S₁/D = 1.521 to 8.621) for Reynolds numbers (Re) from 1000 to 10,000. The novelty of this study lies in developing the first comprehensive set of empirical correlations specifically for LCA tube bundles; these correlations are validated against experimental data with predicted deviations under 10 % for Nu and 15 % for f. Key findings reveal that while LCA tube bundles exhibit moderately lower Nu compared to typical inline tube bundles, they achieve significantly lower friction factors. Consequently, a comparative analysis using the Colburn j-factor to friction factor ratio (j/f) shows the overall thermal-hydraulic performance (j/f) of the LCA tube bundle is 1.2 to 2.65 times greater than that of typical inline tube bundles, primarily attributed to its ultralow flow resistance characteristics. This research provides crucial, validated design tools and quantitatively demonstrates the superiority of the LCA tube bundles for applications prioritizing low pressure drop alongside effective heat transfer, facilitating the design of ultralow flow resistance compact heat exchangers.